Using nano-cathodoluminescence performed in scanning transmission electron microscope (STEM-CL), we have investigated a photonic-bandgap-crystal (PBC) laser structure at T = 17 K. In cross-sectional STEM images the full device structure is clearly resolved. The most dominant luminescence originates from the 3-fold MQW of the active region. The MQW shows a distinct peak wavelength change in growth direction indicating different structural and/or chemical properties of the individual quantum wells. In detail, a clear shift from 427 nm to 438 nm from the first to the top QW is observed, respectively.
Light sources for applications in quantum information, quantum-enhanced sensing and quantum metrology are attracting increasing scientific interest. To gain inside into the underlying physical processes of quantum light generation, efficient photon detectors and experimental techniques are required to access the photon statistics. In this work, we employ photon-number-resolving (PNR) detectors based on superconducting transition-edge sensors (TESs) for the metrology of photonic microstructures with semiconductor quantum dots (QDs) as emitters. For the PNR analysis, we developed a state of the art PNR detection system based on fiber-coupled superconducting TESs. Our stand-alone system comprises six tungsten TESs, read out by six 2-stage-SQUID current sensors, and operated in a compact detector unit integrated into an adiabatic demagnetization refrigerator. This PNR detection system enables us to directly access the photon statistics of the light field emitted by our photonic microstructures. In this contribution, we focus on the PNR study of deterministically fabricated quantum light sources emitting single indistinguishable photons as well as twin-photon states. Additionally, we present a PNR-analysis of electrically pumped QD micropillar lasers exhibiting a peculiar bimodal behavior. Employing TESs our work provides direct insight into the complex emission characteristics of QD- based light sources. We anticipate, that TES-based PNR detectors, will be a viable tool for implementations of photonic quantum information processing relying on multi-photon states.
In this work we show successful metalorganic vapor phase epitaxy (MOVPE) of an AlN/AlGaN distributed Bragg reflector (DBR) that is wavelength matched to GaN quantum dots (QDs) in an AlGaN lambda cavity on top. Full insight into the growth of these structures enables the epitaxy of resonant cavity deep UV single photon emitters.
The DBR was grown on an AlN/sapphire template. In order to obtain a high reflectivity as well as a sufficiently large stopband width, the refractive index contrast needs to be maximized. Additionally, the absorption of QD emission in the high gallium containing layer needs to be minimized. A compromise was found for nominal Al-concentration of 70 % in the AlGaN layers. The resulting DBR splits up into self-organized AlN/Al(X)Ga(1-X)N/Al(Y)Ga(1-Y)N trilayers, which add up to desired lambda/2-periods. Therefore, the stopband at 272 nm with a width of 6 nm shows a maximum reflectivity of 99.7 %.
GaN QDs were obtained by growth of GaN on AlGaN for 10 s with a V/III-ratio of 30 followed by a growth interruption of 30 s. The QDs exhibit sharp emission lines with a FWHM down to 1 meV in µ-PL measurements. The main intensity of the QD ensemble emission is in the range of 250 nm to 275 nm.
Finally, spatially resolved low temperature CL measurements show resonant DBR-enhanced GaN QD emission at 271 nm showing successful wavelength match between a AlN/AlGaN deep UV DBR and GaN QDs in an AlGaN lambda-cavity on top.
We present an alignment procedure which allows for precise gluing of a structure with an optically pumped quantum emitter to the end face of zirconia ferrule with a specially fabricated high numerical aperture single-mode fiber. The proposed method is an important step towards building a single-photon source based on an InGaAs quantum dot emitting in 1.3 μm range and located deterministically in a microlens fabricated by in-situ electron beam lithography and plasma etching to improve the photon extraction efficiency. Since single QDs are very dim at room temperature which hinders QD-fiber adjustment by maximizing the collected photoluminescence signal, the developed method uses light back-reflected from the top surface of the sample with microlens as a feedback signal. Using this approach, we were able to position the high-NA fiber over the center of the microlens with an accuracy of about 150 nm in a lateral direction and 50 nm in a vertical direction. The alignment accuracy was confirmed by following the room temperature emission from quantum wells embedded in a reference microlens. We also present initial low temperature tests of the coupling system mounted in a compact and portable Stirling cryocooler.
Due to its large band gap and excellent electrical properties, nitride-based heterostructures are rapidly becoming a material of choice for RF and power switching applications. However, these devices require a carbon or iron doped semi-insulating buffer to deliver high breakdown voltages and suppress off-state leakage currents. We have grown semi-insulating GaN using precursor-based metal-organic chemical vapor phase epitaxy by intentionally introducing carbon and iron impurities with doping concentration ranging from 1x10^17cm-3 to 5x10^18cm-3 to compensate residual donors. Scanning probe microscopy techniques, scanning surface potential microscopy (SSPM) and bias dependent electric force microscopy (EFM) are mainly used to compare contact potential differences and local potential mapping at the vicinity of dislocation regions. For reference n-type GaN layers doped with Si and Ge, and p-type GaN layers doped with Mg are also investigated. Skew and edge type dislocation densities are estimated from tilt and twist x-ray diffraction measurements using omega-scans for the (0002) reflection and grazing incidence in-plane geometry for the (101 ̅0) reflection. The obtained values are in the range of low 108 cm-2 for screw-type and low 109 cm-2 for edge-type dislocations, independent of doping type and concentration. Locally probing dislocations by SSPM reveals a negative charge contrast with respect to the surrounding areas in C-doped samples increasing with doping concentration whereas Fe-doped samples exhibit no contrast. By investigating the contact potential by EFM, the combined effects of Fermi-level position and surface band bending due to surface states are determined. With the references of n-type and p-type GaN samples, the acceptor states introduced by carbon cause Fermi-level pinning below midgap position whereas acceptor-states by Fe impurities have to be energetically above midgap position. In vertical transport measurements, C-doped GaN layers with a dopant concentration of 4.6x10^18 cm-3 exhibit an up to 5 orders of magnitude lower dark current at room temperature and significantly higher thermal activations than Fe-doped samples with a comparable dopant concentration. In conclusion C-doped samples show superior properties in comparison to Fe-doped samples.
We systematically studied the desorption induced GaN/AlN quantum dot formation using cathodoluminescence spectroscopy directly performed in a scanning transmission electron microscope (STEM). The GaN films were grown by metal organic vapor phase epitaxy (MOVPE) on top of an AlN/sapphire-template. After the deposition of a few monolayers GaN at 960°C a growth interruption (GRI) without ammonia supply was applied to allow for quantum dot formation. A sample series with GRI durations from 0 s to 60 s was prepared to analyze the temporal evolution systematically. Each quantum dot (QD) structure was capped with AlN grown at 1195°C.
Without GRI the cross-sectional STEM images of the reference sample reveal a continuous GaN layer with additional hexagonally-shaped truncated pyramids of 20 nm height and ~100 nm lateral diameter covering dislocation bundles. Spatially averaged spectra exhibit a broad emission band between 260 nm and 310 nm corresponding to the continuous GaN layer. The truncated pyramids exhibit only drastically reduced CL intensity in panchromatic images.
Growth interruption leads to desorption of GaN resulting in smaller islands without definite form located in close vicinity to threading dislocations. Now the emission band of the continuous GaN layer is shifted to shorter wavelengths indicating a reduction of GaN layer thickness. By applying 30 s GRI these islands exhibit quantum dot emission in the spectral range from 220 nm to 310 nm with ultra narrow line widths. For longer growth interruptions the QD ensemble luminescence is shifted to lower wavelengths accompanied by intensity reduction indicating a reduced QD density.
We describe recent work on InGaN lasers and AlGaN UV LEDs at the Palo Alto Research Center (PARC). The
presentation includes results from InGaN laser diodes in which the usual epitaxial upper cladding layer is replaced with
an evaporated or sputtered non-epitaxial material, such as indium tin oxide, silver, or a silver-palladium-copper alloy [1,
2]. Non-epitaxial cladding layers offer several advantages to long wavelength InGaN laser diodes, such as eliminating
the need to expose vulnerable InGaN active layers to the high temperatures required for growing conventional p-AlGaN
cladding layers subsequent to the active layer growth.
The presentation also discusses our recent results on AlGaN UV LEDs. UV LEDs with 300 micron square geometries
operating at λ = 325 nm exhibit output powers of 13 mW with differential quantum efficiencies of 0.054 W/A measured
under wafer-level, unpackaged condition with no heat sink. LEDs operating at λ = 290 nm under similar test conditions
display output powers of 1.6 mW for large-area 300 μm X 1 mm devices.
Results for long-wavelength emitters are presented for semi-polar InGaN/AlGaN/GaN heterostructures grown on
GaN(1122)/m-sapphire templates by metalorganic chemical vapor deposition. The semi-polar GaN layers were 10 to 25
μm thick and grown by HVPE on sapphire substrates. X-ray diffraction measurements indicated high crystallographic
quality that approaches that of GaN(0001) layers on sapphire. A comparison based on optical pumping experiments,
low- and high-density excitation photoluminescence experiments, and atomic force microscopy is drawn between
InGaN/GaN quantum well laser heterostructures grown by metalorganic vapor phase epitaxy either on either polar
GaN(0001)/c-sapphire or on semi-polar GaN(1122)/m-sapphire. C-plane InGaN/GaN/sapphire structures exhibited low
threshold pump power densities < 500 kW/cm2 for emission wavelengths up to 450 nm. For laser structures beyond 450
nm the threshold pump power density rapidly increased resulting in a maximum lasing wavelength of 460 nm. Semipolar
InGaN/GaN(1122)/m-sapphire structures showed a factor of 2-4 higher threshold pump power densities at
wavelengths below 440 nm which is partly due to lower crystalline perfection of the semi-polar GaN/sapphire templates.
However, at longer wavelengths > 460 nm the threshold power density for lasing of semi-polar heterostructures is less
than that for c-plane heterostructures which enabled rapid progress to demonstration of lasing at 500 nm wavelength on
semi-polar heterostructures. The absence of V-type defects in semi-polar, long-wavelength InGaN/GaN structures which
are usually present in long-wavelength c-plane InGaN/GaN structures is attributed to this phenomenon.
Presently VCSELs covering a significant spectral range (840-1300 nm) can be produced based on quantum dot (QD)
active elements. Herein we report progress on selected QD based vertical-cavity surface-emitting lasers (VCSELs)
suitable for high-speed operation. An open eye diagram at 20 Gb/s with error-free transmission (a bit-error-rate < 10-15)
is achieved at 850 nm. The 850 nm QD VCSELs also achieve error-free 20 Gb/s single mode transmission operation through multimode fiber without the use of optical isolation. Our 980 nm-range QD VCSELs achieve error free transmission at 25 Gb/s at up to 150°C. These 980 nm devices operate in a temperature range of 25-85°C without current or modulation voltage adjustment. We anticipate that the primary application areas of QD VCSELs are those that require
degradation-robust operation under extremely high current densities. Temperature stability at ultrahigh current densities,
a forte of QDs, is needed for ultrahigh-speed (> 40 Gb/s) current-modulated VCSELs for a new generation of local and storage area networks. Finally we discuss aspects of QD vertical extended-cavity surface emitting lasers with ultra high power density per emitting surface for high power (material processing) and frequency conversion (display) applications.
We have studied the modulation properties of a vertical cavity surface-emitting laser (VCSEL) coupled to an
electrooptical modulator. It is shown that, if the modulator is placed in a resonant cavity, the modulation of the light
output power is governed predominantly by electrooptic, or electrorefraction effect rather than by electroabsorption. A
novel concept of electrooptically modulated (EOM) VCSEL based on the stopband edge-tunable distributed Bragg
reflector (DBR) is proposed which allows overcoming the limitations of the first-generation EOM VCSEL based on
resonantly coupled cavities. A new class of electrooptic (EO) media is proposed based on type-II heterostructures, in
which the exciton oscillator strength increases from a zero or a small value at zero bias to a large value at an applied
bias. A EOM VCSEL based on a stopband-edge tunable DBR including a type-II EO medium is to show the most
temperature-robust operation. Modeling of a high-frequency response of a VCSEL light output against large signal
modulation of the mirror transmittance has demonstrated the feasibility to reach 40 Gb/s operation at low bit error rate.
EOM VCSEL showing 60 GHz electrical and ~35 GHz optical (limited by the photodetector response) bandwidths is
realized.
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